Jet Ski® and Sea-Doo®

Motorbikes that can ride on water—how cool is that? Jet Skis and Sea-Doos (two popular brand names for what are collectively called Personal Water Craft or PWCs)
are among the fastest and most maneuverable boats of all. That's why
lifeguards and marines use them. A PWC isn't like a normal boat,
powered by an outboard motor and a
propeller.
Nor is it like a motorbike, where the
gasoline engine turns the back wheel.
Instead, a PWC moves along by squirting a
high-powered jet of water behind it. The power of the water squirting
backward pushes the PWC forward. That's the power of science for
you—but how exactly does it work?

Photo: A Sea-Doo Personal Water Craft (PWC) sitting on a trailer
waiting to be launched on the waves. Note the motorcycle handlebars and wing mirrors. Note also how much bigger
a PWC looks when it's on land. The whole of the lower section (colored black) sits beneath the water.

The science of PWCs

Newton's laws

The science behind PWCs was first figured out nearly 350 years ago
by a brilliant Englishman named Isaac Newton (1643–1727). You might not
have thought about PWCs before, but you'll already know about Newton
and his science from party balloons. Everyone's done that trick where
you blow a balloon up till it's almost ready to burst... then release
it so it whizzes round the room. It's always good for a laugh at
Christmas time—but did you know there was solid science behind it?
The science is called Newton's third law of motion.

Around 1666, Isaac Newton set out his three laws of motion—three
simple rules that explain how things move:

Things stay still or travel at the same speed unless something (a
force) pushes or pulls them. Pretty obvious really: a ball stays on the
ground until you kick it.

When a force pushes or pulls an object that's moving, it makes the
object speed up or slow down. The more the force pushes or pulls, the
move the object speeds up or slows down. This is another pretty obvious
one: the harder you kick the ball, the faster it flies through the air.

When a force pushes or pulls something, another force just as big
pushes or pulls in the opposite direction. This is the most confusing
of Newton's laws. It means that if you kick a ball, the ball kicks you
back!

Photo: Science in action: This Sea-Doo is using basic laws of physics (Newton's laws and the conservation of momentum) to propel itself through the water.

Action and reaction

Newton's third law is also called "action and reaction" and you
sometimes see it written like this: for every action (or force), there
is always an equal and opposite reaction (a force of the same size
going the opposite way). It sounds counter-intuitive, but it's
perfectly true. Think about it. If you're on a skateboard and you want
to go forward, you kick backward. The backward kick (the
"action") makes you go forward (the equal and opposite "reaction"). If
you're in the sea and you want to
swim forward using freestyle (crawl),
you pull backward with your arms. The backward pulling force of your
arms (the "action") makes you go forward (the equal and opposite
"reaction"). Space rocket engines and
airplanejet engines also work by
action and reaction. In each case, the force of the hot gas rushing
backward from the engine hurls the rocket or airplane forward through
the air.

People find the idea of action and reaction very confusing. Let's say you're swimming freestyle in the ocean
and you pull backwards on the water with your arms. Now there's clearly an action force here (you pull backwards on the water),
but if there's an equal and opposite reaction force, why don't these two forces simply cancel out?
How come you go anywhere at all? The answer is that the action and the reaction act on different things.
The action is you pulling back on the water. The reaction is your body moving through the water.
The action is a force acting backwards on the water; the reaction is a force acting forwards on your body.
The forces don't cancel out because they act on different things.

Action and reaction explains how a PWC works. The key to a PWC is a small pump with a rotating part called an impeller. When you crank the throttle, the pump sucks in water through a grate underneath the craft and the impeller blasts it out of a hole at the back, so the force of the jet pushing backward (action) drives the whole craft forward (reaction).

Conservation of momentum

Why does the jet need to exit at such high speed? A large PWC can weigh up to about
450kg (1000lbs—about as much as six adults), which is much more than the weight of the water shooting out of it.
A law of physics called the conservation of momentum tells us that the momentum
(mass × velocity) of the water jet firing backward must be equal to the momentum of the craft (and its passengers) going forward, so
to get the PWC moving quickly, the water jet has to exit at immense speed. That's why PWCs need really powerful engines.

How a PWC works

Here's a very simplified cutaway of what's going on inside a typical PWC:

Water is sucked in through a large intake grate on the bottom of the craft.

Power is provided by a medium-sized gasoline engine fired by electric ignition (you switch on by turning a key).
A large PWC might have a 1500cc, four-stroke, four-cylinder engine, which is roughly as big as you'd get in a subcompact car (a small hatchback) or a large motorbike. It has a large 75 liter (20 gallon) fuel tank—to reduce the risk of running out of fuel in the middle of the ocean!

In a car or a motorbike, the engine drives the wheels. In a PWC, the engine's job is to power the water pump and its impeller. An impeller is like a propeller fitted completely within a pipe so it sucks water in at one end of the pipe and blows it out of the other end as a high-speed jet. In a PWC, the impeller has three blades made of stainless steel and it's about 15cm (~5inches) in diameter. Some of the water sucked in is also used to cool the engine.

The water exits through a steerable nozzle at the back of the craft. It's somewhat smaller than the water intake—and that's what builds up the water speed.

Steering a PWC is as easy as steering a motorbike: you just turn
the handlebars to go one way or the other. Instead of turning the
front wheel, as on a motorbike or bicycle, the handlebars pull on a cable (shown here as a yellow curved line) that swivels the water jet to one side or the other, making the whole craft turn at an angle. Because the steering is provided by the power of the water jet, a PWC steers most effectively at high speeds and least effectively when it's going very slowly (when it sometimes barely steers at all).

Photo: PWCs like this Sea-Doo use an impeller to squirt water through a big hole at the back. The exit hole swivels from side to side when you tilt the handlebars. Compare this photo with the ones
up above and you'll see about half the body of a Sea-Doo is permanently underwater. That gives them a low
center of gravity—essential if you want to do amazing maneuvers without toppling over.

Who invented the PWC?

Artwork: Clockwise from the top, three different PWC designs by Theodore Drake,
Clayton J. Jacobson, and Julius Hamori. Pictures courtesy of US Patent and Trademark Office with
colors added for clarity.

Clayton J. Jacobson is the man generally credited with inventing the modern PWC in the 1960s, although it's possibly unfair to give credit to a single inventor. (Who, for example, invented the boat, the fiberglass hull, or the propeller—three other inventions on which Jacobson's PWC was based?) If you flick your way through the records of the US Patent and Trademark Office, you'll find quite a few personal watercraft, including the three interesting examples I've selected in the illustrations here. I've used the same color scheme to quickly give you the essence of each machine: red shows the main body of the craft; gray indicates the steering (handlebars in each case); green shows the seating area or back section; and blue shows the engine, motor, or propeller (and fuel tank).

On the top left, you can see Theodore Drake's Aquatic Device, patented on June 16, 1942, which is a bit like a modern sit-down PWC, except the power is provided by a built-in engine and propeller, similar to an outboard motor, just in front of the driver. Underneath that, we have Julius Hamori's Water Ski Skooter, patented July 30, 1968. Taking its inspiration from traditional water-skis and hydrofoils, it's designed to tilt backward and plane over the surface of the waves. Power is provided by an inboard motor and propeller in the blue section at the back. On the right of the figure, we can see two drawings from Clayton Jacobson's 1969 patent for a Power-driven aquatic vehicle (granted February 11, 1969). It contains all the key features you'll find in a modern PWC, including an internal combustion engine, handlebar steering, water-jet power, and a two-part hull that sits partly above and partly below the water (enabling you to stand up while you drive).

Kid's Stuff! Build yourself a model Jet Ski® or Sea-Doo®

Here's a fun exercise for younger readers—and the young at heart...

You will need

One long, sausage-shaped balloon

One old washing liquid container (or other plastic bottle)

A bathtub or kitchen sink filled with a little water.

Build your boat

Photo: Here's one way to cut your plastic bottle. Remove a section of the top and sides to make a compartment where your balloon will sit, then poke the open neck of the balloon through the neck of the bottle and inflate it so the bulk of the balloon is trapped inside what remains of the bottle. I'm sure you can come up with something even better!

Cut away the bottom of the washing liquid container with scissors. The plastic can be tough to cut, so if you're a young person you may want to ask an adult to help you.

Cut away part of the side of the container too.

Push the balloon into the container.

Inflate the balloon with air. Pinch the neck closed to keep the air in.

Launch your boat in the bathtub (or sink) and watch it go!

Now fill the balloon with water instead of air and launch it again. Does it go better or worse?

Find out more

On this website

Books for younger readers

Jet Ski by Luke Thompson. Paw Prints, 2008. A basic 48-page introduction, including some history and science.

Books for older readers

RYA Personal Watercraft Handbook by Harry Styles. Royal Yachting Association, 2011 (reprinted 2017). A clear, well-illustrated guide that explains everything from checking your PWC before use, launching it, standing up on it for the first time, and using it safely.

Build yourself a jet-drive aquaplane by John Rogers, Popular Science, June 1964. This old article describes how to make your very own personal watercraft using wood, paint, screws, glue—and a commercial jet drive unit.